Spatially homogenizing optical modulator

ABSTRACT

Apparatus of the type including a light source directing a light beam through a measurement zone to a detector, wherein movement of the beam in a measurement direction with respect to the detector gives an indication of the measurement made in the zone, the improvement wherein the position of the beam with respect to the measurement zone is modulated through a preselected amplitude in a modulation direction, the amplitude being independent of light beam movements at the light source or in the path of the beam between the source and the modulating means, whereby the measurement can be made substantially independent of the beam movements in the modulating direction.

BACKGROUND OF THE INVENTION

This invention relates to light-beam deflection instruments, such asrefractometers.

In such instruments, a light source typically directs a beam through atest cell to a detector (e.g., a photocell). A change in some physicalquantity (e.g., refractivity) causes the light beam to move with respectto the detector.

A difficulty is that a change in the position of the light source (e.g.,from movement of the filament in an incandescent bulb) often cannot bedistinguished from a change in the physical quantity being measured, asboth cause movement of the light beam with respect to the detector.

SUMMARY OF THE INVENTION

I have discovered that the position of the light source can bestabilized with respect to the zone (e.g., test cell) in which themeasurement is made by modulating the light beam position (e.g.,cyclically sweeping it back and forth) through a preselected amplitudewhich is independent of light source movement or other uncontrollablemovements of the light beam (such as from thermal eddies). Light sourcemovements only vary the phase of the modulation (e.g., the time at whichthe cyclical sweeping starts and finishes), and the electronics whichprocess the detector output can easily be designed to ignore such phasechanges, such as by determining the average position at which the beamstrikes the detector. The invention provides greatly reduced sensitivityto light source movements, and can be inexpensively implemented inpreferred embodiments.

In some preferred embodiments, the modulation is in the same directionas the beam moves during a measurement; modulation is achieved using arotating prism with its axis of rotation perpendicular to the modulationdirection; two sides of the prism are opaqued to extinguish the lightbeam momentarily during the modulation; a concave reflective surface(e.g., a mirror) behind an incandescent bulb forms the light source, andthe filament of the bulb extends along the axis of rotation of theprism; a randomized fiber-optic bundle transmits the light beam along apath between the modulating device and the measurement zone, with thelight beam being swept back and forth across the inlet to the bundle;and the measurement zone includes a cell for measuring the refractivityof a fluid contained in the cell.

In other preferred embodiments, the light beam is modulated in twodirections to stabilize the position of the light source in these twodirections; the rotating prism has curved sides which act as lenses; andthe light source includes a bulb with a reflective coating on itsexterior surface.

In a second aspect, the invention features an unfocused light path in adirection perpendicular to the direction that the light beam movesduring a measurement. Light from many points of the light source canthereby be spread evenly across this unfocused direction, thereby makingthe measurement insensitive to variations in brightness of the lightsource along this direction. In preferred embodiments, lens means areprovided that focus generally only in the measurement direction; maskingmeans are provided to block light rays from the source such that lightfrom individual points on the source is spread at the detector intolines which substantially all overlap at the detector; and the maskingmeans is positioned adjacent the measurement zone so as to passsubstantially only light passing through the zone.

PREFERRED EMBODIMENT

The structure and operation of a preferred embodiment of the inventionwill now be described, after first briefly describing the drawings.

DRAWINGS

FIG. 1 is a perspective view of said embodiment.

FIG. 2 is a partially cross-sectional view of said embodiment.

FIG. 3 is a cross-sectional view at 3--3 of FIG. 2, showing thephotocell end of the optical bench.

FIG. 4 is a cross-sectional view at 4--4 of FIG. 2, showing the flowcell end of the bench and the outer insulating cylinder and shields,with internal heat shield/light baffle 77 removed.

FIG. 5 is a cross-sectional view at 5--5 of FIG. 2, showing the lightsource.

FIG. 6 is a schematic of the heat exchanger plumbing.

FIGS. 7a and 7b are cross-sectional views through the sample andreference heat exchangers, respectively.

FIG. 8 is a cross-sectional view at 8--8 of FIG. 4, showing constructionof the flow cell.

FIG. 9 is an elevation view of the back surface of the flow cell at 9--9of FIG. 8.

FIGS. 10a and 10b are diagrammatic views of the optical path throughsaid embodiment. [The detector end views at the far left of FIG. 10ahave been rotated 90°.]

FIG. 11 is a block diagram of the electronics that process the outputsof the photocells.

FIGS. 12a and 12b are schematics of the electronic circuits that nullthe photocell output and process the nulled output for display andintegration.

STRUCTURE

Turning to FIG. 1, optical bench 10 is supported inside an oven on fourinsulating posts 12 attached to floor 14 of the oven. Light source 16for the bench is positioned below the bench and outside of the oven.Fiber-optic cable 18 carries light from source 16 to the bench. Sampleliquid from the outlet of a chromatographic column (not shown)positioned inside the oven flows into the optical bench through inlettube 20 (0.009 inch ID), and out through outlet tube 22 (0.040 inch OD).A small diameter sample inlet tube is used to minimize band spreading inthe chromatogram. Similarly, reference liquid flows into and out of thebench through inlet tube 24 (0.020 inch ID) and outlet tube 26 (0.040inch OD). All four tubes are stainless steel and have 1/16 inch outsidediameters. The outlet tubes have larger internal diameters than those ofthe inlets to lower backpressure and its effect on refractivity. Outlettubes 22, 26 are connected together downstream of the optical bench toequalize sample and reference pressures at the flow cell. Electricalwires 28 from photocells 52 (FIG. 2) lead from the bench to processingcircuits shown in FIGS. 11, 12a, and 12b.

Turning to FIGS. 2 through 4, optical bench 10 consists of an innercylinder 32, through which a light beam B is passed, and a concentricouter cylinder 34, which provides an insulating air gap 36. Two flatshields 38 (FIG. 1) retard radiation of heat to and from the bench, andacts as legs (FIG. 4) to support the cylinders, via four bolts 40, onposts 12. End caps 42, 44 close each end of outer cylinder 34, and endcaps 46, 48 each end of inner cylinder 32. End cap 46 supports elongatedoutlet 50 (0.050 inches wide by 0.35 inches high) of fiber-optic cable18 and photocell 52. End cap 48 supports flow cell 54 via cell bridges56, 58, which are attached to the cap and each other by screws andepoxy. Sample inlet and outlet tubes 20, 22 terminate at bridge 58;reference tubes 24, 26 terminate at bridge 56. Recess 60 in end cap 48behind the bridges contains about four coils of sample inlet tube 20.Notches 62, 63 in inner cylinder 34 provide entryways for the tubes. Theend caps, cylinders, and shields are all made from aluminum, to speedwarm up of the bench while also insulating the bench by virtue of airgap 36 between the cylinders.

Turning to FIGS. 8 and 9, flow cell 54 has two hollow chambers 70, 72,for the sample and reference liquids, respectively. Each chamber has atriangular (about 45×45×90 degrees, 0.062 inches on each short side)cross section (FIG. 8), and is connected to its respective inlet andoutlet tubes by internal passages 73. The height (or vertical dimensionin FIG. 9) of the chambers is about 0.50 inches. The flow cell ismanufactured by fusing together, without adhesive, pieces ofborosilicate glass. Teflon seals 75, compressed against the cell by thebridges, provide a seal between the sample and reference tubes andinternal passages 73 of the cell. The front surface 74 of the flow cellis ground to provide an integral lens that has curvature in horizontalbut not vertical planes. The back surface of the cell has a reflectivesurface coating 78 of gold to provide a mirror to reflect light backthrough chambers 70, 72 to photocell 52. The focal line of the lens ispositioned at photocell 52, and the spacing between mirror 78 andphotocell 52 is about 6.0 inches. As shown in FIG. 9, the mirror coating78 is limited to approximately the area directly behind chamber 72,thereby to limit reflection principally to light passing through thetriangular chambers. Other light is absorbed by black epoxy coating 76applied over and around the mirror coating. The mirror coating isslightly larger than the chambers to accommodate variations in theinternal size of chambers 70, 72. The coating stops short of the top ofchambers 70, 72 (FIG. 9) so as not to reflect light passing through thetop of the chambers, where bubbles might form.

To reduce radiant and convective heat transfer to the flow cell fromwithin the optical bench, a blackened disk 77 with rectangularlight-beam aperture 79 (just large enough to expose the flow cell) ispositioned ahead of the flow cell. This disk also serves as a lightbaffle, and is tilted down 10° (FIG. 2).

Sample and reference liquid are brought into the flow cell throughsample and reference counterflow heat exchangers 90, 91 (FIGS. 1, 6, and7), each of which are formed by bonding corresponding inlet and outlettubes together inside a tubular jacket. Each bonded pair is then routedalong a multi-zone path beginning outside the bench and ending at theflow cell. As shown in FIG. 7, sample heat exchangers 90 is constructedby placing tubes 20, 22 inside a tubular copper braid 80, heat shrinkinga polyethylene tube 82 over the outside of the copper braid, and fillingthe interstices between the braid and the inlet and outlet tubes with alow-viscosity, moderately-heat-conductive epoxy 84 (Stycast 3051).Reference tubes 24, 26 are bonded without a copper braid by insertingthe tubes inside a Teflon tube and filling the tube with the samelow-viscosity epoxy as used for the sample tubes. The braid is omittedbecause less efficient heat transfer is needed for the reference, as itdoes not flow during measurement, but only during flushing betweenmeasurements.

Turning to FIG. 6, the multi-zone path followed by the heat exchangersis shown diagrammatically. The first zone for both sample and referenceheat exchangers begins outside the optical bench and extends along theoutside length of the bench between outer cylinder 34 and shields 38(total zone length about 8 inches). The sample heat exchanger 90 ispositioned on the side of the bench closer to the center of the oven,where temperatures are better controlled. At end cap 42, both heatexchangers turn 180° and enter gap 36 between cylinders 32, 34, througha slot (not shown) in the end cap. The second zone for both sample andreference extends along gap 36 (total length about 7 inches). Thereference heat exchanger goes directly from gap 36 into cylinder 32through notch 62 in the end of cylinder. Inside cylinder 32, thereference inlet and outlet tubes are brought directly to flow cell 54via bridge 56.

The sample heat exchanger 90 continues into a third zone beyond the endcap, where it is bent into coil 92, consisting of four turns (total coillength about 24 inches) positioned in the space behind end cap 48. Thelast coil is adjacent to the back of the end cap. From the coils thesample heat exchanger enters cylinder 32 through notch 63 (FIG. 4).Inside the cylinder, sample outlet tube 22 is connected directly to theflow cell. Sample inlet tube 20 is wound in another coil 94 (totallength about 12 inches) before entering the flow cell. Coil 94 ispositioned in recess 90, and potted with a heat-conductive epoxy toprovide good conductivity with the end cap and cell bridges.

Turning to FIGS. 2 and 5, light source 16 includes an incandescent bulb100 (Phillips 6336, H3 base, 6 V, 55 W, operated at 4.8 V) withvertically-extending filament 101, a concave light-focusing mirror 102(gold-coated glass), and a rotating prism 104. The prism is rotated atabout 50 to 60 rpm along an axis parallel to the filament axis by ashaded-pole AC motor 106. The motor also drives a fan 108 which suppliescooling air to the bulb. Prism 104 is about 0.37 inches high, is made ofglass, and has a rectangular cross section. Two opposite surfaces 110 ofthe prism are clear and about 0.3 inches wide. The other two surfaces112 are made opaque with a white opaque silicone rubber, and are about0.25 inches wide. Fiber-optic inlet 114 is round (about 0.150 inches indiameter) and is positioned opposite the prism from the bulb. Mirror 102is positioned so as to focus an image of filament 101 on the face ofinlet 114. Bulb 100 has a peak output in the near infra-red spectrum ata wavelength of about 1000 nanometers.

Fiber optic cable 18 is broken internally into sub-bundles and thesub-bundles are intentionally disordered at one end to randomize thelight path between inlet 114 and outlet 50.

Photocell 52 has two adjacent triangular dual photo-voltaic cells 180,182 (gold-bonded silicon) arranged so that their long dimensions extendhorizontally, which is the direction of movement of the light beam. Eachtriangle is about 0.150 inches long and 0.05 inches high. The spacingbetween the triangles is about 0.008 to 0.010 inches. The shuntimpedance at operating temperature (about 150° C.) is maximized, as isthe sensitivity to long wavelengths.

The oven in which the optical bench resides is heated byproportionally-controlled electrical resistance elements. Within theoven, temperatures can vary as much as 5° to 7° C. from point to point,but by much less (e.g., 0.3° C.) at the same point over time. The timeperiod during which the resistance elements are on is varied inproportion to the difference between the actual oven temperature and thedesired temperature and in proportion to the integral of thisdifference. To make oven temperature less sensitive to variations in ACline voltage, the time period is also made inversely proportional to thesquare of the line voltage, as the heat generated by the elements isproportional to the square of the line voltage. The elements are SCRcontrolled, and are turned on and off only at zero crossings of current.

FIGS. 11, 12a, and 12b show the electrical circuits that process theoutputs of photocell 52. FIG. 11 shows the overall circuitry in blockdiagram form. Panel inputs 118 (e.g., recorder gain) are fed to acentral processor 120. The central processor (CPU) initiates theautomatic electrical zeroing (nulling) of the photocell outputs, andsends signals via buffers 122 and gain latches 124 to circuitry shown inFIG. 12b to set the gain for display of the chromatogram on a recorder.An analog data acquisition voltage (D.A.V.) is converted to digital andsent by the processor via the input buffers to a panel display 126.

FIG. 12a shows the circuitry for electrical zeroing. The current outputs(AC signals) of photocells 180, 182 are brought via shielded cable tocurrent-to-voltage converters 130. The AC voltages A, B produced by theconverter are summed and amplified by a gain of 2.2 at amplifier 132, toform the expression -2.2 (A+B), which is called SUM. Amplifier 134subtracts voltage A from voltage B, and adds to the difference the sumof three voltages: SUM, FINE ZERO, and COARSE ZERO. The latter twovoltages are produced by multiplying SUM by a negative scale factor.Thus the output of amplifier 134 (ZEROED OUTPUT) can be expressed as

    [B-A]-2.2[0.33-0.67K.sub.C -0.0033K.sub.F ][A+B]

where K_(C) is the coarse zero scale factor and K_(F) is the fine zeroscale factor. Scale factors K_(C), K_(F) are set between about zero andabout one by the digital circuitry of block 136, whenever a signal issent across the AUTOZERO COMMAND lead. Normally zeroing would be donebefore a chromatogram was generated, but can be done at any time.

The above expression for the ZEROED OUTPUT can be presented insimplified form as

    [B-A]-K[A+B]

where K is the overall scale factor. The expression is independent ofvariations in the overall brightness of the light beam strikingphotocell 52 because the zeroing term (K[A+B]) is not a constant, but,like the difference term (B-A) is proportional to beam brightness. Forexample, if the brightness were to rise by 10%, both the difference termand the zeroing term would similarly rise by 10%, and thus the wholeexpression would still remain equal to zero. When beam deflection doesoccur, as the result of refractivity changes, the zeroing term remainsroughly constant because of the complementary shape of the two cells180, 182, which at any horizontal location have roughly the samecombined vertical height.

Two successive-approximation registers 138, 140 drive a pair ofdigital-to-analog converters 142, 144 to form the FINE ZERO and COARSEZERO signals. Each of converters 142, 144 multiplies the SUM signal by ascale factor set by the digital output of registers 138, 140. Registers138, 140 follow a conventional successive approximation algorithm toselect the digital outputs or scale factors. About once a second, theregisters receive a clock pulse from chip 148, which produces a slowclock from the much faster processor clock signal. At each clock pulse,the output of a register is adjusted in response to the output ofcomparator 146 which indicates whether the applied FINE/COARSE ZEROsignal is too large or too small. The input to comparator 146 is the DCOUTPUT, produced at filter amplifier 150 (FIG. 12b). A FILTER RESETconnection between the zeroing circuitry and filter amplifier 150 isused during the zeroing process to discharge capacitors in the filterand reset the DC OUTPUT to zero. This allows for a more rapid autozerosequence. Register 138 works first to set the coarse scale factor K_(C),and then register 140 to set the fine scale factor K_(F). The AUTOZEROCOMMAND is used by the CPU to start the autozero sequence. TheAUTOZEROING signal is used to alert the central processor that therefractometer is autozeroing.

Turning to FIG. 12b, there is shown circuitry for processing the ZEROEDOUTPUT. Amplifier 152 raises or lowers the signal level in response tocommand signals 151 from the central processor 120 via the data latch124. Demodulator 154 (with the help of phase computing block 153)converts the AC signal to DC, and filter amplifier 150 smooths the DCsignal. Switching block 156 operates during zeroing to turn off theRECORDER and INTEGRATOR signals. It also is used to change the polarityof the DC signal in response to a POLARITY signal from the centralprocessor 120 via data latch 124. Downstream of block 156 the DC signalis processed by amplifier 158, and supplied to an integrator outputlead. The DC signal is also processed by attenuator 160, under controlof the central processor via signals 162. The attenuator produces arecorder output 164, which is supplied to a recorder output terminal andto amplifier 166, and a data acquisition voltage (D.A.V.), which issupplied to the central processor for panel display. Block 168 suppliesa mark signal for the recorder in response to the AUTOZERO COMMAND, toindicate on the chromatogram the point at which the sample injectionoccurs. The CPU issues the AUTOZERO COMMAND at the time of sampleinjection.

OPERATION

In operation, the oven surrounding the optical bench and chromatographiccolumn is turned on, and about an hour and one half warm up period isallowed for temperature equalization within the bench. After warm up,solvent is pumped through the sample and reference circuits within thebench. When solvents are changed, sufficient time is allowed forflushing both circuits. Flow is then stopped in the reference circuit(but reference chamber 72 remains filled with reference liquid). Asample is then injected into the sample column. The electrical output ofthe refractometer is zeroed by initiating the automatic zeroing sequencedescribed above. Sample passes through the chromatographic column andinto the optical bench. Generally speaking, variations in refractivityof the sample cause movement of the light beam with respect to photocell52, and thereby change the electrical output, which is plotted againsttime on a chart recorder, producing a chromatogram.

Temperatures within chambers 70, 72 of the flow cell are maintainedwithin about 0.0001° C. of each other during operation to minimizeerror. A temperature difference between the two flow cell chambersresults in a refractivity difference. Temperature equalization isachieved by providing good thermal insulation around the flow cell, inthe form of air gap 36 between the inner and outer cylinders, shields38, and blackened disk 77; surrounding the flow cell with a thermalmass, in the form of bridges 56, 58 and end cap 48; and directingincoming sample flow through a very efficient counterflow heat exchangerto bring the temperature of the sample to the flow cell temperature.Incoming sample upstream of the heat exchanger is typically as much as1° C. (and possibly 2° to 3° C.) different in temperature than the flowcell because of spatial differences in oven temperature and because ofheat generated by viscous heating inside the inlet tube. This differencein temperature is gradually reduced along the length of the heatexchanger by thermal conduction between the inlet and outlet tubes. Atthe end of the heat exchanger, whatever very small temperaturedifference remains is minimized by heat transfer between end cap 48 andcoil 94 just prior to entry into the flow cell.

The sample heat exchanger is divided into three zones to improve itsefficiency, with each successive zone being more thermally stable andcloser to the temperature of the flow cell. The construction of the heatexchanger provides good thermal conduction between tubes but very lowconduction along the flow direction of the tubes. There is significantheat transfer between the tubes and the surrounding air; thus thermalinteraction between the heat exchanger and the region surrounding itmust be considered. The first zone, between outer cylinder 34 and shield38, provides a gradual approach in temperature before the heat exchangerenters the optical bench. The length of this zone is greater than 10% ofthe length of the sample inlet tube within the bench. Without the firstzone, i.e., if the inlet and outlet tubes were joined just outside theentry to end cap 42, there would be a steeper approach in temperaturealong the heat exchanger, and much of this approach in temperature wouldoccur along portions of the heat exchanger inside air gap 36, therebyundesirably transferring heat to or from the bench. With the preferredarrangement of a first zone outside the bench, the heat exchangertemperature is closer to that of the bench when entering the air gap.

The heat exchanger enters the gap at the photocell end of the bench,thereby assuring that whatever heat transfer to or from the bench doesoccur is at a location well separated from the flow cell.

This same concept of routing the heat exchanger through increasinglymore thermally stable regions is also applied to the second and thirdzones. In the second zone, the sample heat exchanger is directed alongair gap 36 from the photocell end to the flow cell end, wheretemperature stability is highest. In the third zone, the sample heatexchanger is coiled behind the flow cell end cap, with each successivecoil being closer to the end cap and flow cell.

As a final step, the sample inlet tube alone is coiled in recess 60 ofend cap 48 to minimize whatever small temperature difference remainsbetween the incoming sample and the flow cell.

Because the reference solvent does not flow during a measurement, thereference heat exchanger is less sophisticated. It lacks the thirdcoiled zone, and has no copper, heat-conductive braid to surround inletand outlet tubes. Limited heat exchange is provided on the referenceside to maintain rough temperature equalization during flushing of thereference circuit, thereby shortening the period needed to stabilizetemperatures after flushing.

The optical elements of the refractometer are shown diagrammatically inFIGS. 10a and 10b. For clarity the optical path is shown unfolded, withmirror 78 treated as a window. FIG. 10a shows a horizontal sectionthrough the optical path; FIG. 10b shows a vertical section. Thedetector end views at the far left of FIG. 10a have been rotated 90°.

Turning to FIG. 10a, a single light ray B is shown to illustrate beammovements. Lens surface 74 on flow cell 54 focuses the light emergingfrom fiber-optic cable outlet 50 onto photocell 52. The focused image onthe photocell is shown diagrammatically in the views on the left side ofthe Figure. To illustrate the effect caused by rotation of prism 104,four views (A through D) of the prism in different angular positions areshown along with the corresponding positions of the light beam.

Light passing through chambers 70, 72 is bent in proportion to thedifference in the refractive index of the liquids in the two chambers.Referring to FIG. 8, the chambers are conventionally constructed so thatsurface 190 in chamber 70 is parallel to surface 188 in chamber 72 and,similarly, so that surfaces 184 and 186 are parallel. These foursurfaces are the four at which light is bent by refraction. If theliquid of the same refractive index is in both chambers, light will bebent by the same amount at each of the corresponding parallel surfaces,and will emerge from the flow cell along a path B which is essentiallyunaffected by changes in refractivity common to both chambers. If liquidin the two chambers differs in refractive index, light will be bentdifferentially at these parallel surfaces, and will emerge along a pathskewed from the equal-refractivity path. Such a condition is illustratedin FIG. 10a by light ray B'. The amount by which the light beam isskewed or bent at the flow cell is measured by detecting the position ofthe image of the beam at photocell 52. The difference between theelectrical outputs of the two triangular cells 180, 182 can very finelyresolve the horizontal position of the light beam. Imperfections inalignment of the photocell with the flow cell and other tolerances inthe system typically cause these electrical outputs of the two cells tobe unequal even when sample and reference liquids have the samerefractive index. This initial electrical difference is nulled by theautomatic zeroing procedure described above.

Ideally, the light beam location on the photocell 52 should only be afunction of the difference in refractive index between sample andreference (and not a function of the location of bulb filament 101). Toachieve this, the light intensity distribution across the fiber opticoutlet 50 must be spatially stable over the time period ofchromatographic interest (1 second to several hours). This requires thatthe light intensity distribution into the fiber optics be stable. Asviewed from the fiber optics inlet 114, the apparent position of bulbfilament 101 varies due to filament distortion and thermal eddies in theair path between the filament and the inlet. Filament movements alongthe length of the filament (vertical in FIG. 2) are relativelynoncritical. Similarly, changes in the filament distance from the fiberoptics inlet are not observable and thus are noncritical. Along thethird axis of movement (vertical in FIG. 5) the apparent filamentlocation as viewed by the fiber optics inlet must be spatiallystabilized for the beam location at the photocell to be independent offilament location. To achieve stabilization, a Spatially HomogenizingOptical Modulator (SHOM) in the form of rectangular prism 104 isemployed in the light path between the filament and the fiber opticsinlet. The prism provides an optical path offset which is a function ofits rotation position. When the prism rotates, the filament opticallyappears to sweep across the face of the fiber optics inlet 114. Inposition A, the prism is so oriented that the light from filament 101 isbent outside the acceptance angle of the fibers in cable 18, andnegligible light is transmitted to the bench. In position B, the prismhas rotated sufficiently for light to be transmitted through at leastsome of the fibers in the cable. In position C, the prism has swept thefilament image across the face of the fiber optics inlet. In position D,the prism has moved the image to a position beyond the acceptance angleof the fibers, and again negligible light is transmitted. As the prismrotates further, the beam first reappears beyond the acceptance angle ofthe fibers, as in position A, and then another sweep begins. Thesweeping action, including the period of negligible light transmission,occurs two times during each revolution of the prism, or about 100 timesper second.

If filament 101 moves or appears to move, this has the effect ofchanging the time at which the beam starts and finishes its sweep acrossthe fiber optics inlet. That is, only the phase of the beam movement isaltered by movement of the filament. The electronics described abovecompute the average or middle position swept by the image. Theelectronics are insensitive to such phase or time shifts, and thus theundesirable effects of filament shift are minimized.

The apparent light source position is further stabilized by using therandomized fiber optics bundle 18. In a perfectly randomized fiberoptics bundle, adjacent fibers at one end of the bundle are randomlydistributed at the other end. Therefore, increasing the light on oneside of the bundle input while decreasing it on the other side resultsin no change in the light distribution across the fiber optics outputend. In actual practice, the randomization in a bundle is not perfect,and some change does occur at the output end. But using the randomizedfiber optics does further decrease the effect of filament motion on beammovement at photocell 52.

As can be seen in FIG. 10b, the optics do not focus the beam onto thephotocell in the vertical direction, as done in the horizontaldirection. Instead, light emerging from outlet 50 of cable 18 remainsunfocused in vertical planes, thereby producing for each point of lightat the outlet a vertical line of light at the photocell. The verticalheight of this line is limited by the vertical height of mirror 78,which acts as a mask. Light rays from individual points, e.g., points Xand Y, on the cable outlet 50 fan out, but only rays inside of limitrays X₁, X₂ (Y₁, Y₂ for point Y) reach the photocell. (Other rays arenot reflected through the photocell). The vertical heights of mirror 78,photocell 52, and cable outlet 50 and the spacing between the flow celland photocell ends of the bench are all selected so that the limit raysfor all points on the cable outlet strike fully above and fully belowtriangular cells 180, 182 of the photocell. Limit rays for point X andpoint Y, at the top and bottom extremities of the cable outlet, areshown in FIG. 10b. Thus each point on the cable outlet produces a lineof uniform intensity at the photocell. And these lines all overlap overthe photocell, thereby assuring a uniform vertical intensity across thephotocell no matter what vertical variation in intensity may exist atthe cable outlet (e.g., due to variation in filament intensity in thevertical direction). The end result is the light intensity profile shownat the left side of FIG. 10b. Across the vertical height of thephotocells the intensity is uniform; outside the photocells theintensity falls off to zero. Vertical uniformity of light intensity atthe photocells is needed to linearly determine the horizontal light beamlocation on the triangular-shaped cells 180, 182. (A vertical variationin intensity would be indistinguishable from a horizontal movement ofthe light beam.)

OTHER EMBODIMENTS

Other embodiments are within the scope of the invention. For example,fiber optics cable 18 could be eliminated so that light emerging fromthe SHOM is passed directly to the flow cell; rotating prism 104 couldinclude one or more curved lens surfaces for focusing light; a secondrotating prism could be used to stabilize the position of the lightsource in a second direction (e.g., orthogonal to the directionstabilized in the first prism); different mirror arrangements could beused to reflect light toward the prism and optical bench (e.g., acoating of gold on portions of the light bulb surface); and a rotatingmirror could be substituted for the rotating prism.

OTHER INVENTIONS

Subject matter relating to the integral mirror coating on the flow cellwas the invention of Lawrence J. Finn (U.S. application Ser. No. 051,810filed June 25, 1979).

Subject matter relating to the integral lensing surface on the flow cellwas the sole invention of William W. Carson (U.S. application Ser. No.051,811 filed June 25, 1979).

Subject matter relating to zeroing the photocell output was the jointinvention of William W. Carson and Norman F. Rolfe (U.S. applicationSer. No. 051,809 filed June 25, 1979).

Subject matter relating to resetting the filter during zeroing was theinvention of Norman F. Rolfe (U.S. application Ser. No. 059,292 filedJuly 20, 1979).

Subject matter relating to the heat exchanging between an inlet and anoutlet tube was the joint invention of William W. Carson and John S. Roe(U.S. application Ser. No. 050,326 filed June 20, 1979).

What is claimed is:
 1. In apparatus of the type including a light sourcedirecting a light beam through a measurement zone to a detector, whereinmovement of said beam in a measurement direction with respect to saiddetector gives an indication of the measurement made in said zone, theimprovement comprising:means for modulating, through a preselectedamplitude in a first modulation direction, the position of said beamwith respect to said measurement zone, said amplitude being independentof light beam movements at said light source or in the path of said beambetween said source and said modulating means, whereby said measurementcan be made substantially independent of said beam movements in saidmodulation direction.
 2. The improvement of claim 1 wherein said firstmodulation direction is said measurement direction.
 3. The improvementof claim 1 wherein said modulation cyclically sweeps said beam between afirst and a second position along said first modulation direction. 4.The improvement of claim 3 wherein said modulating means includes meansfor substantially extinguishing said beam momentarily at said first andsecond positions.
 5. The improvement of claim 1 wherein said modulatingmeans is located along the path of said light beam ahead of said zone.6. The improvement of claim 5 wherein said modulating means comprisesoptical modulating means for modulating said beam.
 7. The improvement ofclaim 6 wherein said optical modulating means comprises a rotatingoptical element which bends the path of said beam by refraction.
 8. Theimprovement of claim 7 wherein said rotating optical element comprises amultisided prism rotating about an axis perpendicular to said modulationdirection.
 9. The improvement of claim 8 wherein said prism has foursides.
 10. The improvement of claim 8 wherein said sides are planar. 11.The improvement of claim 1 or 8 further comprising reflection meansbehind said light source for reflecting light toward said measurementzone and said modulating means.
 12. The improvement of claim 11 whereinsaid reflection means comprises a concave, mirrored surface.
 13. Theimprovement of claim 12 wherein said light source is a bulb and saidconcave, mirrored surface is external from said bulb.
 14. Theimprovement of claim 8 wherein said light source is an incandescent bulbwith a filament extending along an axis parallel to said axis of prismrotation.
 15. The improvement of claim 8 wherein some of said sides ofsaid prism are mode opaque to cause said light beam to be momentarilyextinguished during intervals of each period of said modulation.
 16. Theimprovement of claim 1 or 14 wherein said measurement zone comprises arefractometer cell at which said beam is bent by changes in refractivityof a fluid.
 17. The improvement of claim 1 further comprising afiber-optic bundle interposed between said modulating means and saidlight source for transmitting said beam, said modulation being withrespect to the inlet of said bundle.
 18. The improvement of claim 17wherein said bundle is broken into subbundles that are disorderedrandomly between the inlet and outlet of said bundle.
 19. Theimprovement of claim 1 further comprising means for determining theaverage position at which said beam strikes said detector.
 20. Inapparatus of the type including a light source directing a light beamthrough a measurement zone to a detector, wherein movement of said beamin a measurement direction with respect to said detector gives anindication of the measurement made in said zone, the improvementcomprisingmeans for permitting said light beam to pass unfocused inselected planes from said light source through said measurement zone tosaid detector, wherein said means includeslens means for focusing saidlight beam generally only in said measurement direction and maskingmeans for blocking portions of said light beam so that light fromindividual points on said source is spread at the detector into lines oflight at the detector, whereby variation in the intensity of said lightsource along the unfocused direction does not substantially affect themeasurement at said detector.
 21. The improvement of claim 20 whereinsaid masking means includes a mask adjacent said measurement zone forpassing only light passing through said zone.